Second harmonic generation with schiff bases

ABSTRACT

Devices for and method of generating coherent second harmonic light radiation. The devices comprise a laser source of coherent light radiation at a fixed fundamental frequency,a Schiff base compound, means for directing the output radiation of the laser onto the Schiff base compound, and output means for utilizing the second harmonic frequency. Preferably, one moiety of the Schiff base compound has a hyperpolarizable group and the other moiety of the Schiff base compound has a configuration so that the compound crystallizes in a non-centrosymmetric configuration.

This is a division of application Ser. No. 825,469 filed Feb. 3, 1986,now U.S. Pat. No. 4,733,109.

TECHNICAL FIELD

This invention is concerned with materials for nonlinear optical devicesfor the conversion of optical energy at one frequency to optical energyat another frequency.

BACKGROUND OF THE INVENTION

Laser techniques have been developed to a great extent and it ispossible at present to obtain various kinds or types of laser light byutilizing solid, gas, and liquid media. In many purposes of applicationsutilizing coherent laser light, laser light having various wavelengthsis required and in some cases, a laser light exhibiting a continuousspectrum over a certain range of wavelengths is required. However, it isdifficult to satisfy the aforesaid purposes or requirements even withthe various kinds of laser light obtainable at present. Hence, a meanshas frequently been employed in which coherent laser light is convertedinto laser light of the second harmonic; that is to say, laser light ofa wavelength of which the frequency is obtained is twice the fundamentalfrequency, by passing the laser light through a nonlinear opticalcrystal. Conversion of the wavelength of a laser light by a parametriceffect using such a nonlinear optical crystal has also been investigatedand developed.

In the prior art, monocrystalline forms of potassium dihydrogenphosphate (KDP), ammonium dihydrogen phosphate (ADP), barium sodiumniobate (BaNaNbO₃), and lithium niobate (LiNbO₃) were used as coherentlight phase modulators and for generating higher frequency harmonics.Monocrystalline KDP and ADP, while offering greater resistance tooptical irradiation induced surface damage due to laser beambombardment, do not exhibit large optical nonlinearities. This renderedthese crystals unsuitable for higher harmonic frequency generation orconversion. In contrast, BaNaNbO₃ and LiNbO₃ showed large nonlinearitiesbut, unfortunately, a low resistance to optical damage. In this regard,the term "resistance to optical damage" means the number of times thesurface of a crystalline material can be bombarded (shots) with laserrediation of a given power density in watts per unit area before thesubject crystal shows signs of opacity. Thus, a crystal showing highresistance would require a larger number of shots than a crystal of lowresistance for the same power density of the incident laser beams.

All of the above-mentioned crystals vary their refractive index when anelectric field is applied to them. Thus, such crystals find employmentas electro-optic phase modulators. While these crystals can be used bothin the form of powdered samples and large single crystals, it has beenrecognized that large single crystals are preferred because largeelectro-optical and nonlinear effects can be obtained through their use.

The possibility of using organic molecules in nonlinear optical deviceshas generated much interest recently because a large number of moleculesare available for investigation. Some substituted aromatic molecules areknown to exhibit large optical nonlinearities. The possibility of suchan aromatic molecule having large optical nonlinearities is enhanced ifthe molecule has donor and acceptor groups bonded at opposite ends ofthe conjugated system of the molecule.

U.S. Pat. No. 4,199,698 discloses that the nonlinear optical propertiesof 2-methyl-4-nitroaniline (MNA) make it a highly useful material innonlinear devices that convert coherent optical radiation including afirst frequency into coherent optical radiation including a secondfrequency. The nonlinear devices have means for introducing coherentradiation of a first frequency into the MNA and means for utilizingcoherent radiation emitted from the MNA at a second frequency.

U.S. Pat. No. 4,431,263 discloses that diacetylenes and polymers formedfrom diacetylenic species, which are amendable to close geometric,steric, structural, and electronic control, provide nonlinear optic,waveguide, piezoelectric, and pyroelectric materials and devices.Diacetylenes which are crystallizable into crystals having anoncentrosymmetric unit cell may form single crystals or be elaboratedinto a thin film upon a substrate by the Langmuir-Blodgett technique.Such films may be polymerized either thermally or by irradiation for usein nonlinear optical and other systems. Diacetylenes are covalentlybonded to substrates through the employment of silane species andsubsequently polymerized to yield nonlinear optic and other deviceshaving high structural integrity in addition to high efficiencies andoptical effects.

Sov. Phys. Crystallogr. 22, 305 (1977) discloses that a compound havingthe structure ##STR1## exhibits second harmonic generation. However,this paper does not suggest or predict that Schiff base compounds ingeneral will exhibit second harmonic generation. Isomers of theaforementioned compound, e.g. ##STR2## pack in a centrosymmetric spacegroup, and, consequently, observation of second harmonic generation iseliminated.

SUMMARY OF THE INVENTION

In one aspect this invention involves generation of coherent secondharmonic light radiation by devices that incorporate certain Schiff basecompounds therein. In another aspect, this invention involves a methodof generating coherent second harmonic light radiation by theaforementioned devices. In a third aspect, several of the Schiff basecompounds found to be useful in the devices described herein are novelper se, i.e. they are not known to ever have been previously prepared.

In general, second harmonic generators of this invention comprise, incombination, a laser source of coherent light radiation at a fixedfundamental frequency, an organic molecular crystalline compoundselected from particular classes of Schiff base compounds, means fordirecting the output radiation of the laser onto the organic molecularcrystalline Schiff base compound, and output means for utilizing thesecond harmonic frequency.

Schiff base compounds that can be used in the present invention aredefined as those Schiff base compounds which crystallize in anon-centrosymmetric configuration and which contain a moiety selectedfrom the group consisting of (a) 4-dialkylaminophenyl methylene amino,(b) 2-hydroxyphenyl methylene amino, and (c) 4-hydroxyphenyl methyleneamino.

The following four classes of Schiff base compounds are preferred foruse in devices for generation of coherent second harmonic lightradiation:

(I) Compounds containing 4-dialkylaminophenyl methylene amino moietybonded to a substituted phenyl moiety, wherein the substituent orsubstituents on the phenyl moiety is selected so that the Schiff basecompound crystallizes in a non-centrosymmetric configuration;

(II) Compounds containing 4-dialkylaminophenyl methylene amino moietybonded to a second moiety selected such that the Schiff base compound isoptically active, such as, for example, wherein the second moietycontains an asymmetric carbon atom;

(III) Compounds containing 2-hydroxyphenyl methylene amino moiety bondedto a moiety which renders the compound capable of being a photochromicmaterial;

(IV) Compounds containing 4-hydroxyphenyl methylene amino moiety bondedto an unsubstituted or substituted 4-nitrophenyl moiety, wherein anysubstituent or substituents on the 4-nitrophenyl moiety, if any, isselected so that the Schiff base compound crystallizes in anon-centrosymmetric configuration.

In general, it is preferred that one moiety of the Schiff base compoundhave a hyperpolarizable group and the other moiety of the Schiff basecompound have a configuration which provides that the resultant compoundcrystallizes in a non-centrosymmetric configuration.

The Schiff base compounds of the present invention are useful forproviding second optical effects. These compounds can be incorporated indevices known to be useful as second harmonic generators.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawing is a diagrammatic presentation of a suitable arrangement fordemonstrating the second harmonic generating properties of the Schiffbases of this invention.

DETAILED DESCRIPTION

All Schiff base compounds are characterized by the anil linkage,##STR3## Schiff base compounds that have been found to exhibit secondharmonic generation are crystalline in form, and are preferably in solidcrystalline form. While Schiff base compounds in liquid crystalline formmay also exhibit second harmonic generation, detection of signals fromSchiff base compounds in liquid crystalline form is difficult.Regardless of whether they are in solid crystalline form or in liquidcrystalline form, Schiff base compounds suitable for this invention musthave a non-centrosymmetric configuration. Non-centrosymmetric speciesare those which have no center of symmetry on either the molecular orcrystalline unit cell level.

Schiff base compounds suitable for this invention must be sufficientlytransparent to incident electromagnetic radiation in the specified rangeso that the efficiency of second harmonic generation (SHG) can bedetected by conventional SHG detection devices. For example, where thewavelength of incident radiation is 1.064 μm, and the wavelength of thegenerated second harmonic is 0.532 μm, the Schiff base compound mustallow sufficient radiation to pass at both wavelengths to detect atleast one photon of 0.532 μm wavelength light.

Schiff base compounds that have been found to be preferable for thisinvention can be represented by the following general formulas: ##STR4##wherein R¹ represents a member selected from the group consisting ofchloro, bromo, iodo, cyano, lower alkoxy, and lower alkylthio, and

R² represents a member selected from the group consisting of halo,cyano, lower alkoxy, lower alkylthio, and hydrogen, provided that R¹ isnot the same as R² ; ##STR5## wherein R³ represents a group whichrenders the molecule optically active, and

R⁴ represents a member selected from the group consisting ofdimethylamino and diethylamino; ##STR6## wherein R⁵ represents a groupwhich renders the molecule capable of being a photochromic material,e.g. alkylphenyl, halophenyl, cyanophenyl, nitrophenyl; ##STR7## whereinR⁶ represents a member selected from the group consisting of hydrogen,lower alkoxy, lower alkyl, lower alkylthio, cyano, and halo.

As used herein, "photochromic materials" are materials which changetheir color as a result of absorption of electromagnetic energy.Photochromism is discussed in Kirk-Othmer Encyclopedia of ChemicalTechnology, Third Edition, Vol. 6, John Wiley & Sons (N.Y.: 1984) pp.121-128. See also J. Chem. Soc. 2041 (1964). "Optically active" meansthat characteristic of a substance which renders it capable of rotatingthe plane of vibration of polarized light to the left or right. SeeHackh's Chemical Dictionary, Fourth Edition, McGraw-Hill, Inc. (N.Y.,1972). "Lower alkoxy" means alkoxy group having 1to 4 carbon atoms;"lower alkylthio" means alkylthio group having 1 to 4 carbon atoms;"lower alkyl" means alkyl group having 1 to 4 carbon atoms; "halo" meansthe group consisting of fluoro, chloro, bromo, and iodo.

In structural formula II, R³ is preferably a group of the structure##STR8## wherein C* represents an asymmetric carbon atom that is bondedto the nitrogen atom of the anil linkage of the Schiff base compound,and

R^(a), R^(b), and R^(c) independently represent a member of the groupselected from hydrogen, alkyl, aryl, aralkyl, alkaryl, provided thatR^(a), R^(b), and R^(c) are all different.

Preferably R^(a) represents unsubstituted or substituted phenyl group,R^(b) represents unsubstituted or substituted alkyl group, and R^(c)represents hydrogen atom. The particular substituents on R^(a) and R^(b)should be selected so that the Schiff base molecule is optically active.Other than this requirement, the character of the substituents is notcritical. It should also be noted that if the R-isomer of the Schiffbase provides a non-zero value of SHG efficiency, the S-isomer of theSchiff base will also provide a non-zero value for SHG efficiency.

While many of the Schiff base compounds represented by theaforementioned structural formulas are known, several, including some ofthose exhibiting the highest values of SHG efficiency, have never beforebeen reported. These novel Schiff base compounds can be represented bythe following formulas: ##STR9## wherein R¹ represents member selectedfrom the group consisting of chloro, bromo, iodo, cyano, lower alkoxy,and lower alkylthio, and

R² represents a member selected from the group consisting of halo,cyano, lower alkoxy, and lower alkylthio, provided that R¹ is not thesame as R² ; ##STR10## wherein R⁶ represents a member selected from thegroup consisting of hydrogen, lower alkoxy, lower alkyl, loweralkylthio, cyano, and halo.

It has been found that, in general, substitution of a bulky,electron-rich substituent in the ortho-position (2-position) on theN-phenyl ring of a Schiff base compound (i.e., the phenyl moiety bondedto the nitrogen atom of the anil linkage) provides a Schiff basecompound that exhibits efficient second harmonic generation.Additionally, the further substitution of a second bulky, electron-richsubstituent in the 5-position on the N-phenyl ring of a Schiff basecompound provides improvement in second harmonic generation efficiency.

Methods of preparing Schiff base compounds are well known. They can bereadily prepared by condensation of an aldehyde or ketone with anappropriate amine under a variety of conditions.

The reactants can simply be introduced into a monohydric alcoholsolvent, e.g. methanol, ethanol, and allowed to react in the presenceof, or absence of, external heat. Alternatively, the reactants can beintroduced into a non-alcoholic solvent, e.g. toluene, and allowed toreact in the presence of, or absence of external heat. Water generatedby the reaction can be removed preferably by means of absorption bymolecular sieves. According to a different procedure, the reactants canbe introduced into toluene, heated, and the water removed therefrom bymeans of a Dean-Stark trap. A catalyst, e.g. p-toluenesulfonic acid, canbe employed, but is not required.

The Schiff base reaction product can be purified by recrystallizationfrom solvents, e.g. ethanol, methanol, acetone, acetonitrile,methylethyl ketone, acetone/hexane, acetone/isopropanol, methylenechloride/heptane. Selection of the optimum solvent is well-known tothose of ordinary skill in the art.

The aforementioned Schiff base compounds are substantially transparentto electromagnetic radiation having wavelengths from 400-500 nm to 1064nm. Accordingly, they are useful in second harmonic generators whereinboth incident radiation and emergent radiation range from 400 nm to 1064nm.

Devices that are capable of generating coherent second harmonic lightradiation with the Schiff bases described herein are well known in theart. Representative examples of such devices are described in U.S. Pat.Nos. 3,395,329, 3,431,484, and 3,858,124, all of which are incorporatedherein by reference for the purpose of describing devices which canincorporate the Schiff bases described herein and exhibit efficientsecond harmonic generation by means of such incorporation.

Schiff base crystals were evaluated for SHG efficiency using the SHGpowder test described in Kurtz et al., J. Appl. Phys. 39, 3798, 1968.Each Schiff base was ground and sieved and then mixed with a fluid,i.e., a liquid, to minimize refraction caused by differences in theindex of refraction between the particles and the ambient atmosphere.The index-matched sample was placed between cell flats spaced 0.2 mmapart. Particles having mean diameter greater than 90 μm but less than180 μm were used. Each sample was mixed with a drop of index matchingfluid (Cargille n=1.63 or n=1.58 fluids or n=1.631 Convalex oil). Thesamples were not indexed matched critically, so that the actual SHGefficiencies reported in the examples may be higher.

Referring now to FIG. 1, infrared radiation at 1064 nm from a Q-switchedNd-YAG laser 10 was weakly focused onto the cell 12 containing theprepared sample. In the device illustrated in FIG. 1, the means fordirecting the output radiation of the laser, e.g. a lens, onto the cell12 containing the Schiff base sample is integrated into the laser 10 andis not shown as a separate component. Means for directing the outputradiation of the laser onto the organic molecular crystalline compoundare well-known to one of ordinary skill in the art. An infrared blockingfilter 14 placed behind the sample allowed only the second harmonicfrequency or frequency-doubled radiation to pass through a 1/3 metermonochrometer 16 tuned at 532 nm. The output of the monochrometer 16 wasdirected to a cooled photomultiplier tube 18, and the resulting signalwas processed by a boxcar averager 20 that averages over many laserpulses. Urea was the chosen standard because of its high second ordercoefficient and its ready availability. The urea standard was preparedin the same manner as the samples. The urea standard was indexed matchedreasonably well, with a mismatch of about 0.01. The reported efficiencyof a sample is its SHG signal normalized to that of the urea standardmeasured under the same experimental conditions.

The following examples are meant to illustrate, but not limit thisinvention. Parts and percentages are by weight unless otherwiseindicated. All of the compounds prepared in the examples and comparativeexamples were characterized by standard analytical techniques, e.g.infrared spectroscopy, ultraviolet/visible absorption spectroscopy,nuclear magnetic resonance spectroscopy, melting point, and elementalanalysis.

EXAMPLE 1

Equimolar amounts of the reactants o-cyanoaniline (0.042 mole) andp-dimethylaminobenzaldehyde (0.042 mole) were added to 150 ml toluenecontaining 50 ml molecular sieves in a 250-ml Erlenmeyer flask. Water ofreaction was removed by absorption by the molecular sieves. The reactionmixture was stirred over the molecular sieves at room temperature (20°C.) for 72 hours. The resulting solution was filtered, the toluenefiltrate saved, and the molecular sieves extracted with methylenechloride. The methylene chloride extract was combined with the toluenefiltrate, and the solvent removed therefrom under reduced pressure toprovide a residue which was recrystallized from a solvent mixturecomprising methylene chloride and heptane. The yield was approximately33%. The resulting product was2-[{(4-dimethylaminophenyl)methylene}amino]-cyanobenzene. This compoundexhibited an SHG efficiency of 2.30.

EXAMPLES 2-5

Preparation of the Schiff base compounds listed in Table I below wasaccomplished by the procedure described in Example 1 above, i.e.equimolar amounts of the amine and aldehyde were reacted together andthe resulting product recovered according to the described procedure.

                                      TABLE I                                     __________________________________________________________________________                                                        SHG                       Example                                                                            Aldehyde       Amine    Schiff base            efficiency                __________________________________________________________________________    2    p-dimethylaminobenzaldehyde                                                                  o-bromoaniline                                                                         2-[{(4-dimethylaminophenyl)methylene}amino]-                                  4                      1.87                                                   bromobenzene                                     3    p-dimethylaminobenzaldehyde                                                                  o-iodoaniline                                                                          2-[{(4-dimethylaminophenyl)methylene}amino]-                                                         0.016                                                  iodobenzene                                      4    p-dimethylaminobenzaldehyde                                                                  o-methoxyaniline                                                                       2-[{(4-dimethylaminophenyl)methylene}amino]-                                                         2.3                                                    methoxybenzene                                   5    p-dimethylaminobenzaldehyde                                                                  2-methoxy-5-                                                                           2-[{(4-dimethylaminophenyl)methylene}amino]-                                                         28.5                                          chloroaniline                                                                          4-chloro-methoxybenzene                          __________________________________________________________________________

COMPARATIVE EXAMPLES A-F

Preparation of the Schiff base compounds listed in Table II below wasaccomplished by the procedure described in Example 1 above, i.e.equimolar amounts of the amine and aldehyde were reacted together andthe resulting product recovered according to the described procedure.

                                      TABLE II                                    __________________________________________________________________________                                                        SHG                       Example                                                                            Aldehyde       Amine    Schiff base            efficiency                __________________________________________________________________________    A    p-dimethylaminobenzaldehyde                                                                  o-fluoroaniline                                                                        2-[{(4-dimethylaminophenyl)methylene}amino]-                                  1                      0.001                                                  fluorobenzene                                    B    p-dimethylaminobenzaldehyde                                                                  2-methoxy-5-                                                                           2-[{(4-dimethylaminophenyl)methylene}amino]-                                                         0.0                                           methylaniline                                                                          4-methyl-methoxybenzene                          C    p-dimethylaminobenzaldehyde                                                                  2-methoxy-5-                                                                           2-[{(4-dimethylaminophenyl)methylene}amino]-                                                         0.0                                           nitroaniline                                                                           4-nitro-methoxybenzene                           D    p-dimethylaminobenzaldehyde                                                                  2,4-dibromoaniline                                                                     1-[{(4-dimethylaminophenyl)methylene}amino]-                                                         0.0                                                    2,4-dibromobenzene                               E    p-dimethylaminobenzaldehyde                                                                  2,4-dichloroaniline                                                                    1-[{(4-dimethylaminophenyl)methylene}amino]-                                                         0.0                                                    2,4-dichlorobenzene                              F    p-dimethylaminobenzaldehyde                                                                  m-cyanoaniline                                                                         3-[{(4-dimethylaminophenyl)methylene}amino]-                                                         0.0                                                    cyanobenzene                                     __________________________________________________________________________

EXAMPLE 6

Equimolar amounts of the reactants, R-(+)-α-methylbenzylamine (0.077mole) and salicylaldehyde (0.077 mole), were mixed in 50 ml ethanol, andthe mixture stirred and heated for about 2 hours to insure thecompletion of the reaction. The mixture was then allowed to cool to roomtemperature (20° C.). The product, which crystallized upon cooling, wasisolated by filtration. The yield was approximately 67%. The resultingproduct was R-1-[{(2-hydroxyphenyl)methylene}amino]-1-phenylethane. Thiscompound exhibited an SHG efficiency of 0.18.

EXAMPLE 7-10

Preparation of the Schiff base compounds listed in Table III below wasaccomplished by the procedure described in Example 6 above, i.e.equimolar amounts of the amine and aldehyde were reacted together andthe resulting product recovered according to the described procedure.

                                      TABLE III                                   __________________________________________________________________________                                                        SHG                       Example                                                                            Aldehyde       Amine    Schiff base            efficiency                __________________________________________________________________________    7    Salicylaldehyde                                                                              aniline  [{(2-hydroxyphenyl)methylene}amino]-                                                                 0.21                                                   benzene                                          8    Salicylaldehyde                                                                              m-bromoaniline                                                                         3-[{(2-hydroxyphenyl)methylene}amino]-                                                               0.16                                                   bromobenzene                                     9    Salicylaldehyde                                                                              p-bromoaniline                                                                         4-[{(2-hydroxyphenyl)methylene}amino]-                                                               0.016                                                  bromobenzene                                     10   p-dimethylaminobenzaldehyde                                                                  R-(+)-α-methyl-                                                                  R-1-[{(4-dimethylaminophenyl)methylene}amino]                                 -                      0.97                                          benzylamine                                                                            1-phenylethane                                   __________________________________________________________________________

COMPARATIVE EXAMPLES G-K

Preparation of the Schiff base compounds listed in Table IV below wasaccomplished by the procedure described in Example 6 above, i.e.equimolar amounts of the amine and aldehyde were reacted together andthe resulting product recovered according to the described procedure.

                                      TABLE IV                                    __________________________________________________________________________                                                        SHG                       Example                                                                            Aldehyde       Amine    Schiff base            efficiency                __________________________________________________________________________    G    p-dimethylaminobenzaldehyde                                                                  p-chloroaniline                                                                        4-[{(4-dimethylaminophenyl)methylene}amino]-                                  .                      0.0                                                    chlorobenzene                                    H    p-dimethylaminobenzaldehyde                                                                  p-bromoaniline                                                                         4-[{(4-dimethylaminophenyl)methylene}amino]-                                                         0.01                                                   bromobenzene                                     I    p-dimethylaminobenzaldehyde                                                                  p-cyanoaniline                                                                         4-[{(4-dimethylaminophenyl)methylene}amino]-                                                         0.0                                                    cyanobenzene                                     J    p-dimethylaminobenzaldehyde                                                                  p-methoxyaniline                                                                       4-[{(4-dimethylaminophenyl)methylene}amino]-                                                         0.0                                                    methoxybenzene                                   K    p-dimethylaminobenzaldehyde                                                                  aniline  [{(4-dimethylaminophenyl)methylene}amino]-                                                           0.0                                                    benzene                                          __________________________________________________________________________

EXAMPLE 11

Equimolar amounts of the reactants, o-chloroaniline (0.09 mole) andp-dimethylaminobenzaldehyde (0.09 mole), were dissolved in 200 mltoluene in a 500 ml round-bottomed flask fitted with a condenser andDean-Stark trap for separating and collecting water generated by thereaction. The reaction mixture was heated to reflux and the watercollected. The reaction was allowed to proceed until close to thetheoretical amount of water had been collected. The amount of timerequired varied from 12 to 24 hours. The solvent was removed underreduced pressure and the residue was recrystallized from a solventmixture comprising methylene chloride and heptane. The yield wasapproximately 60%. The resulting product was2-[{(4-dimethylaminophenyl)methylene}amino]-chlorobenzene. This compoundhad an SHG efficiency of 0.67.

COMPARATIVE EXAMPLES L-S

Preparation of the Schiff base compounds listed in Table V below wasaccomplished by the procedure described in Example 11 above, i.e.equimolar amounts of the amine and aldehyde were reacted together andthe resulting product recovered according to the described procedure.

                                      TABLE V                                     __________________________________________________________________________                                                        SHG                       Example                                                                            Aldehyde       Amine    Schiff base            efficiency                __________________________________________________________________________    L    p-nitrobenzaldehyde                                                                          o-methoxyaniline                                                                       2-[{(4-nitrophenyl)methylene}amino]-                                                                 0.0                                                    methoxybenzene.sup.a                             M    p-naphthaldehyde                                                                             o-methoxyaniline                                                                       2-[{(2-naphthyl)methylene}amino]-                                                                    0.002                                                  methoxybenzene.sup.b                             N    p-chlorobenzaldehyde                                                                         o-methoxyaniline                                                                       2-[{(4-chlorophenyl)methylene}amino]-                                                                0.0                                                    methoxybenzene                                   O    p-dimethylaminobenzaldehyde                                                                  o-methylaniline                                                                        2-[{(4-dimethylaminophenyl)methylene}amino]-                                                         0.0                                                    methylbenzene                                    P    p-dimethylaminobenzaldehyde                                                                  o-ethylaniline                                                                         2-[{(4-dimethylaminophenyl)methylene}amino]-                                                         0.0                                                    ethylbenzene                                     Q    p-dimethylaminobenzaldehyde                                                                  m-methoxyaniline                                                                       3-[{(4-dimethylaminophenyl)methylene}amino]-                                                         0.0                                                    methoxybenzene                                   R    p-dimethylaminobenzaldehyde                                                                  m-bromoaniline                                                                         3-[{(4-dimethylaminophenyl)methylene}amino]-                                                         0.0                                                    bromobenzene                                     S    p-diethylaminobenzaldehyde                                                                   2-methoxy-5-                                                                           2-[{(4-diethylaminophenyl)methylene}amino]-                                                          0.0                                           chloroaniline                                                                          4-chloro-methoxybenzene                          __________________________________________________________________________     .sup.a Schiff base compound was recrystallized from a mixture of acetone      and ethanol.                                                                  .sup.b Schiff base compound was recrystallized from a mixture of methylen     chloride and hexane.                                                     

EXAMPLE 12

Equimolar amounts of the reactants, 2-methoxy-5-bromoaniline (0.025mole) were dissolved in 200 ml toluene in a 500-ml round-bottomed flaskfitted with a condenser and containing 50 ml of molecular sieves. Themixture was refluxed for 8 hours, then allowed to cool to roomtemperature (20° C.). The solution was filtered, the toluene filtratesaved, and the molecular sieves extracted with methylene chloride. Themethylene chloride washings were combined with the toluene, and thesolvent was removed under reduced pressure. The residue obtained wasrecrystallized from a solvent mixture comprising methylene chloride andheptane. The yield was approximately 60%. The resulting product was2-[{(4-dimethylaminophenyl)-methylene}amino]-4-bromomethoxybenzene. Thiscompound had an SHG efficiency of 2.9.

EXAMPLES 13-17

Preparation of the Schiff base compounds listed in Table VI below wasaccomplished by the procedure described in Example 12 above, i.e.equimolar amounts of the amine and aldehyde were reacted together andthe resulting product recovered according to the described procedure.

                                      TABLE VI                                    __________________________________________________________________________                                                        SHG                       Example                                                                            Aldehyde       Amine    Schiff base            efficiency                __________________________________________________________________________    13   p-dimethylaminobenzaldehyde                                                                  o-methylthio-                                                                          2-[{(4-dimethylaminophenyl)methylene}amino]-                                                         0.7                                           aniline  methylthiobenzene                                14   p-diethylaminobenzaldehyde                                                                   (R)-(+)-α-methyl-                                                                R-1-[{(4-diethylaminophenyl)methylene}amino]-                                                        4.9                                           benzylamine                                                                            1-phenylethane                                   15   p-hydroxybenzaldehyde                                                                        p-nitroaniline                                                                         4-[{(4-hydroxyphenyl)methylene}amino]-                                                               1.6                                                    nitrobenzene.sup.a                               16   p-hydroxybenzaldehyde                                                                        2-methoxy-4-                                                                           3-methoxy-4-[{(4-hydroxyphenyl)methylene}                                                            10.7                                          nitroaniline                                                                           amino]nitrobenzene.sup.a                         17   p-hydroxybenzaldehyde                                                                        2-methyl-4-                                                                            3-methyl-4-[{(4-hydroxyphenyl)methylene}-                                                            0.3                                           nitroaniline                                                                           amino]nitrobenzene.sup.a                         __________________________________________________________________________     .sup.a Molecular sieves were extracted with N,Ndimethylformamide and the      Schiff base compound was recrystallied from a mixture of                      N,Ndimethylformamide and water.                                          

COMPARATIVE EXAMPLES T-Y

Preparation of the Schiff base compounds listed in Table VI below wasaccomplished by the procedure described in Example 12 above, i.e.equimolar amounts of the amine and aldehyde were reacted together andthe resulting product recovered according to the described procedure.

                                      TABLE VII                                   __________________________________________________________________________                                                         SHG                      Example                                                                            Aldehyde       Amine     Schiff base            efficiency               __________________________________________________________________________    T    p-dimethylaminobenzaldehyde                                                                  2,5-dibromoaniline                                                                      1-[{(4-dimethylaminophenyl)methylene}amino]-                                                         0.04                                                   2,5-dibromobenzene                              U    p-dimethylaminobenzaldehyde                                                                  m-methylthioaniline                                                                     3-[{(4-dimethylaminophenyl)methylene}amino]-                                                         0.0                                                    methylthiobenzene                               V    p-dimethylaminobenzaldehyde                                                                  2-methyl-5-                                                                             2-[{(4-dimethylaminophenyl)methylene}amino]-                                                         0.0                                          chloroaniline                                                                           4-chloro-methylbenzene                          W    p-dimethylaminobenzaldehyde                                                                  2,5-dichloroaniline                                                                     1-[{(4-dimethylaminophenyl)methylene}amino]-                                                         0.0                                                    2,5-dichlorobenzene                             X    p-dimethylaminobenzaldehyde                                                                  2-methoxy-4-                                                                            2-[{(4-dimethylaminophenyl)methylene}amino]-                                                         0.0                                          bromoaniline                                                                            5-bromomethoxybenzene                           Y    m-hydroxybenzaldehyde                                                                        p-nitroaniline                                                                          4-[{(3-hydroxyphenyl)methylene}amino]-                                                               0.0                                                    nitrobenzene.sup.a                              __________________________________________________________________________     .sup.a Schiff base was recrystallized from a mixture of                       N,Ndimethylformamide and water.                                          

From observation of the results of the examples and comparativeexamples, it has been found that Schiff base compounds derived fromp-dimethylaminobenzaldehyde, p-diethylaminobenzadehyde,o-hydroxybenzaldehyde, or p-hydroxybenzaldehyde provide SHG efficiencyvalues above zero.

From observations of the results of the examples and comparativeexamples, it can be seen that not all Schiff base compounds exhibitpositive values of SHG efficiency. For example, in comparing Example 4with Comparative Examples Q and J, it can be seen that when the methoxygroup is in the 2- position (Example 4), the value of SHG efficiency ispositive, but when the methoxy group is in the 3-position (ComparativeExample Q) or 4-position (Comparative Example J), the values of SHGefficiency is zero.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scopeand spirit of this invention is not to be unduly limited to theillustrative embodiments set forth herein.

What is claimed is:
 1. Compound represented by the formula ##STR11##wherein R¹ represents a member selected from the group consisting ofchloro, bromo, iodo, cyano, and lower alkylthio, andR² represents amember selected from the group consisting of cyano and lower alkylthio,provided that R¹ is not the same as R².
 2. Compound represented by theformula ##STR12## wherein R⁶ represents a member selected from the groupconsisting of lower alkoxy, lower alkyl, lower alkylthio, and cyano.